The present invention relates to a method of producing an extremely hydrophobic surface according to the preamble of claim 1, and particularly to a method of producing an extremely hydrophobic surface on to a material, the method comprising directing at a surface to be structured a particle spray structuring the surface so as to structure the surface, and coating the structured surface with a hydrophobic material. The present invention further relates to an apparatus for producing an extremely hydrophobic surface according to the preamble of claim 17, and particularly to an apparatus for producing an extremely hydrophobic surface on to a material, the apparatus comprising a device for directing a particle spray at a surface to be structured so as to structure the surface, and coating means for coating the structured surface.
The structure of a surface plays an important role as far as the properties of several products are concerned. A known example is a Lotus surface wherein a hydrophobic surface is transformed into an extremely hydrophobic one because the surface is provided with bumps that are spaced at distances of 20 to 40 micrometres from one another and, further, because the entire surface contains wax crystals of 200 to 2000 nanometres in size. Most generally it can be stated that by changing the micro/nanostructure of a surface, an initially hydrophilic or hydrophobic surface may be transformed into an extremely hydrophilic or extremely hydrophobic surface. An extremely hydrophilic and/or extremely hydrophobic surface is economically significant when manufacturing self-cleaning surfaces or surfaces that stay easily clean.
In the present context, a superhydrophobic surface is defined as a surface where a contact angle of a water droplet is greater than 120°.
U.S. Pat. No. 3,354,022, published on 21 Nov. 1967, E.I. du Pont de Nemours and Company, discloses a water-repellent surface whose water-repellency is based on the surface being provided with bumps that are spaced at an average distance of at most 1000 micrometers from one another and that have a height of at least half the average distance.
U.S. Pat. No. 6,811,856 B2, published on 2 Nov. 2004, Creavis Gesellschaft für Technologie and Innovation GmbH, discloses a self-cleaning surface with a hydrophobic surface structure formed by means of differences in height on the surface which are produced by particles affixed to the surface. The particles are 20 to 500 nm in size.
In addition to self-cleanability, hydrophobic surfaces are significant in fluidics, particularly when trying to decrease flow resistance of channels. U.S. Pat. No. 6,852,390 B2, published on 8 Feb. 2005, Entegris, Inc., discloses an ultraphobic surface produced by means of uniformly shaped differences of micro or nanoscale from the surface. In Physics of Fluids, vol. 16, no. 12, December 2004, Jia Ou, Blair Perot & Jonathan P. Rothstein, “Laminar drag reduction in microchannels using ultrahydrophobic surfaces”, pp. 4635 to 4643, it is shown that the flow resistance of a laminar flow channel may be substantially reduced if the surface of the flow channel has a uniform microscale structure.
Self-cleaning surfaces are of great economical importance e.g. in windows. U.S. Pat. No. 6,997,018 B2, published on 14 Feb. 2006, Ferro Corporation, discloses a method of micro- and nanostructuring a glass surface. The method is based on affixing inorganic particles having a diameter of less than 400 nm to the surface of the glass when the temperature of a glass product is within a range of 700 to 1200° C.
A surface may be structured either uniformly or nonuniformly, i.e. randomly. In practice, randomly structured surfaces are more relevant since they are less expensive to manufacture. Micro- and nanostructures have been manufactured onto surfaces by several methods, such as lithography, etching, micro-stamping, chemical vapor disposition (CVD), and physical vapor disposition (PVD).
U.S. Pat. No. 6,309,798 B1, published on 30 Oct. 2001, Studiengesellschaft Kohle mbH, discloses a lithographic method for nanostructuring a surface. The lithographic method requires multiphase surface processing, thus not being an advantageous method for structuring large surfaces.
U.S. Pat. No. 6,468,916 B2, published on 22 Oct. 2002, Samsung SDI Co., Ltd., discloses a method of forming a nano-sized surface structure. The method comprises several steps: forming a micro-structure, depositing a carbon polymer layer on top of the micro-structured surface, a first plasma etching process, creating a mask layer, and a second reactive etching. This method thus also requires multiphase surface processing, so it is not an advantageous method for structuring large surfaces.
The method described in U.S. Pat. No. 6,997,018 for structuring a surface of glass is a useful structuring method for large surfaces, but it is limited to structuring a glassy surface at a temperature of more than 700° C.
The prior art methods are incapable of producing an extremely hydrophobic surface onto large surfaces of a material. Furthermore, the known methods and devices are incapable of producing an extremely hydrophobic surface in a controlled and adjustable manner such that it could be utilizable in commercial-scale production. Thus, a need exists for such a method, which method would also be useful and advantageous for implementing large and optically impeccable surfaces.